I will yield to your greater knowledge. I would fertilize in March, May, September and early November. I live on Lake Erie so I actually have different growing seasons for my front and back yard. For example, the wind today is out of the north and the water temperature is ~ 32. so my backyard is freezing. My front yard is 41.I disagree with your comment about fertilizing at least four times/yr, especially if you have a cool season grass like bluegrass or tall fescue. Here in the metro Cincinnati area, which is in the transition zone like Pittsburgh, we put down something like an 8-0-0 with .15% Dimension in the spring. Dimension is good to use as it will kill crab grass up to the first or second tiller. We will come back and apply another application of Dimension only in June if the we are concerned about crab grass in late Aug/early Sept. We never fertilize in the summer unless you are talking about a warm season grass like Zoysia.
For cool season grass, the next fertilizer application should be made in September. We apply 30-3-4, and then again in November closer to Thanksgiving with a 35 -0-3 here in the metro area. We are not concerned about snow mold as we only average 18 inches of snow/year. Some competitors actually apply straight urea 46-0-0 in November.
The thinking was this:
- march for early growth
- May for strength in the middle of peak growing season
- September for fall growth (roots, filling in bare spaces from dog piss and drought)
- November to give the grass a good end/start to the season.